1. Field of the Invention
The present invention relates to a thermal wax transfer printing method suitable for printing a half tone image.
2. Description of Related Background Art
A thermal transfer printing method includes a thermal wax transfer type printing method and a sublimation type printing method. With the thermal wax transfer type printing method, the back surface of an ink film is heated with a thermal head, and melted or softened ink is transferred to a recording paper. With the sublimation type printing method, dye is sublimated into an image reception layer of the recording paper. The former method cannot adjust the amount of ink transfer according to heat energy, so that it is used for printing a binary value image such as characters and lines. The latter method can adjust the amount of dye according to heat energy, so that it is used for printing a half tone image.
Although the conventional thermal wax transfer type printing method cannot print a half tone image, it has various advantages. Namely, there is left only a small trailing line after a printed line as compared to the sublimation type, the trailing line resulting from a thermal hysteresis of a thermal head. The stability of a printed image is large as compared to the sublimation type. Paying attention to the advantages of the thermal wax transfer type printing method, the inventors of this invention developed an improved printing method capable of printing a half tone image having a high gradation by using a plurality of heating elements or resistive elements disposed in the disposing direction or a main scan direction, and by changing the length of a recorded ink dot in a sub scan direction perpendicular to the main scan direction in accordance with the density of a picture element to be recorded.
This improved printing method has been found, however, unsatisfactory. Namely, since the length of a heating element in the sub scan direction is a minimum recording width, it is necessary to narrow the length of a heating element to improve the gradation of a printed image. However, there is a limit in narrowing the length of a heating element in view of the durability and manufacturing cost, resulting in a hardship of high gradation.
Furthermore, with the above-described improved printing method, virtual picture cells each capable of being printed with an ink dot of a maximum size are arranged in a matrix. Therefore, if one line in the main scan direction is recorded in the same density, a plurality of rectangular ink dots each having the same size are printed, thereby forming a line extending in the main scan direction. Therefore, a plurality of conspicuous parallel lines extending in the main scan direction are included in a printed image depending upon the type of an image pattern of an original. An original image having a plurality of parallel lines, particularly, a rattan blind, will produce moire because of a superposition of original parallel lines and the above conspicuous parallel lines. If an original image has oblique lines, the oblique lines will have notches.
In printing a color image by using the above-described improved printing method, a plurality of color ink dots are recorded superposed one upon another in each picture cell as shown in FIGS. 10A to 10C. In FIGS. 10A to 10C, virtual three picture cells in the main scan direction and virtual two picture cells in the sub scan direction are shown by broken lines, totaling six picture cells. The density or size of the picture cell of each color in the main and sub scan directions is set for example to 100 lines/inch (8 ink dots/mm), and the size of a picture cell is for example 125.times.125 .mu.m. FIG. 10A shows cyan ink dots recorded in all six picture cells 2a to 2f as indicated by hatched portions. FIG. 10B shows magenta ink dots recorded in all six picture cells 3a to 3f. FIG. 10C shows yellow ink dots recorded in picture cells 4a, 4b, 4e, and 4f among six picture cells 4a to 4f. If color registration is complete, i.e., if there is no alignment error of colors, for example, picture cells 2a, 3a and 4a take the same position on a recording paper so that the recorded ink dots are superposed one upon another to form a half tone through the subtractive mixture. If color registration is complete, it is possible to print a color hard copy having a high image quality. However, if there occurs a shear of color registration in the sub scan direction, it has been found that the hue of an image will change. For example, the color of a face changes between yellowish and pinkish. Such hue change becomes maximum if the shear of color registration in the sub scan direction is as large as a half (pitch) of a picture cell multiplied by an integer. In order to eliminate hue change and reproduce an original image with high fidelity, the shear of color registration should be suppressed several .mu.m or smaller if a picture cell is 125.times.125 .mu.m. However, a thermal wax transfer type color printing apparatus presently available is difficult to obtain a highly precise alignment of colors.